Growing consumer interest in low-maintenance alternatives to traditional building materials is having a real effect on the residential decking market: By 2011, wood-plastic composites will account for almost a quarter of all residential decking sales, predicts the Freedonia Group, a research firm. That’s up from 4 percent in 1996.

As the popularity of composite decking increases, so too will the need for builders and remodelers to familiarize themselves with the category’s expanding number of products. Each manufacturer offers a distinct blend of polymers, organic fibers, and additives, meaning that material strength, durability, and resistance to mold and fire vary widely according to brand. Some composites manage to look a lot like real wood, while others don’t come close. Some have met code test standards, but most have not. And each product has its own installation guidelines — which tend to be quite a bit more complex than those for traditional wood decking.

Pros & Cons

Manufacturers of composite decking have worked hard to sell homeowners and builders on the long-term advantages of their products. Composites eliminate the expense of regular staining and annual water sealing. They don’t check or split like solid lumber, and they promise to outlast even the most rot-resistant of solid sawn species. In addition, many makers use at least some recycled content, whether for the wood fiber, the plastic, or both — an important selling point for some consumers.

Composites have their drawbacks, too. For example, many consumers have complained of a tendency to mildew, especially when the decking is installed in the shade. Manufacturers contend that any decking product would mildew in shady conditions — but while that may be true, the problem has been a surprise to homeowners who believe they were promised a product that required no maintenance.

To keep decks looking good, most composite decking manufacturers recommend periodic sweeping and washing with water to keep the surface free of debris and dirt, which can support mold growth. If stains or mold do occur, the companies recommend a deck wash containing sodium hypochlorite and detergent.

In cases where the decking has been left wet and poorly maintained for extended periods, fungal decay has been reported with some composites. Fading is another issue. Composites almost always fade when exposed to sunlight, so customers should check weathered samples when choosing colors. Weathering is usually complete in three to six months, depending on exposure.

The weathered sample.

It’s best to show clients a weathered sample when they choose a color of composite decking. These Fiberon boards demonstrate the typical weathering that occurs in three to six months on virtually all composite decking.

The Chemistry

Most wood-plastic composite decking products are made with polyethylene, commonly used for plastic bags and bottles and other consumer products. A smaller number of manufacturers use polypropylene, which is a much harder plastic, or polyvinyl chloride (PVC), also known as vinyl.

While different plastics have different mechanical qualities, a decking product can’t be evaluated solely by the polymer used, since the manufacturer can manipulate the mechanical properties. As Robert Tichy, research engineer at Washington State University, explains, “With the appropriate additives, you could make any polymer perform the way you want. It’s just a question of cost.”

Adding wood fiber to the composite offers several benefits. Besides being cheaper than plastic, wood tends to outperform plastic in strength, rigidity, and flammability tests. Wood experiences less thermal expansion than plastic and usually weighs less. The wood fiber also helps manufacturers more closely approximate the look of traditional wood decking. On the downside, the presence of wood fiber makes the material more porous and vulnerable to stains, and introduces the risk of mold and decay problems.

Chemical additives like flame retardants and UV stabilizers can also be mixed into the formula, as can antimicrobial chemicals intended to fight mold and mildew. Each additive increases the production cost.

Solid Composites

Of all the composites on the market, solid products like Trex most closely resemble wood decking in size and thickness. But as anyone who has handled Trex knows, solid composites lack the stiffness of their sawn-lumber counterparts, typically requiring joist spacing no greater than 16 inches on-center. Diagonal and commercial applications are usually limited to 12-inch spacing.

On the other hand, the material’s flexibility does offer advantages: Because it can be bent more easily than traditional lumber, curves and round sections are easier to incorporate.

Porch flooring. While most solid composites mimic square-edged deck boards, one exception is a solid tongue-and-groove porch flooring product called CorrectPorch (formerly sold as Tendura), made by Correct Building Products. Composed of a polypropylene-hardwood composite similar to the one used in the company’s decking products, CorrectPorch planks have a 3-inch coverage and are butted like traditional solid-sawn porch flooring. They come predrilled 16 inches on-center and have a built-in spacer “bead” that creates a small gap between boards. The spacer was added in response to thermal-expansion problems that plagued earlier versions of the product (a thermal expansion chart can be found on the product Web site, correctporch.com).

A built-in spacer “bead” creates a small gap between planks that allows for drainage and thermal expansion. The composite can be painted or left unfinished.

CorrectPorch solid T&G planks are intended for use on covered porches with sloped floors.

For warranty protection, the flooring must have a roof above, be pitched 1/4 inch per foot away from the house, and be well-ventilated underneath to prevent heat buildup. It can be left natural or primed and painted.